Cavity resonator



June 10, 1952 BANOS, JR 2,600,186

CAVITY RESONATOR Filed Oct. 3, 1945 INVENTOR.

ALFREDO BANOS JR. BY fiw Q/LLQ,

ATTORNEY Patented June 10, 1952 STAYES E A'EEN'E aisles 1 This invention relates .to wavemeters and more particularly to resonant cavities, often termed echo boxes.

An echo box is a structure having a resonant cavity internally surfaced by electrically conductive material. Usually the shape of the cavity i simple, as the modes, or' manners in which eometrically simple cavities resonate in response to electromagnetic'energysupplied' thereto are more easily calculable than for other shapes.

For manyjgeometrical configurations the mode of the resonant oscillationi'and' the resonant frequencies thereof are mathematically computed with ease and known to theartj E In general, echo boxes have many uses in dealing with electromagnetic radiation in the microwave region. Among these may be mentioned employment with radio echo detection apparatus to provide artificial or phantom targets which may be used to tune a receiver when no real targets are conveniently available. have a very high Q, of the order of 100,000, and when excited by pulsed radiation of the nature customarily used in radio echodetection apparatus the cavity will ring or oscillate for several microseconds after the terminaton of 'a transmitted pulse. During the oscillation time, or ringing time, energy is returned to the radio.de tection apparatus to produce signals. Another use of an echo box may'be for tuning a receiver to the echothus returned. One common useis as a wavemeter. A cavity of adjustable size is calibrated, theadjustment at 'a point of maximum amplitude fof -reso nance providing 'a frequency measure, and sucha wavemeter is espe- Echo boxes cially usefulin transm'itter adjustment because receiver tuning need not affect the results. Another common use may be as a high Q circuit.

One especially commonand useful geometrical configurationfor an echo box cavity is a right .cir- I cular cylindrical cavity to which ener ymay be supplied or withdrawn directly through one or more apertures in the walls, or throughpickup loops inserted therethrough, or the like.

Cylindrical cavities may be manufactured with ordinary precision machinery with ease and great accuracy. A diifliculty which has been ,en-

countered heretofore, however, and the cause, for which has been substantially eliminated-by this present invention, as will appear more fully herfeinafter, is that the endplates (thetop and bottom plates) of the cyli'ndrical cavity are parallel to a high degreeofaccuracy, the ,echobox :has a decidedly 7 inferior 1 characteristic .in that the Q; of the;;box; is -ver-y much lowered, and the a gr (o1. its-s44) response or ringing time consequently decreased. This difficulty has required the use of variousa'd justable means top'rovide the desired degree of parallelism between the plane surfaces of the end plates as will be mentioned hereinafter. These adjustments are highly critical, and ra quire electrical tests'usually suitable'tobe performed only at the factory or at a test bench, and sealing and locking of the adjustable means after the'test. The possibilities of th'e adjustment being impaire'd'by inechanical'vibrations, temperature changes, and'neces'sary'handling of the box, and the further requirement of strict tolerances prove a burden mthemansfa-cmre and use of cylindrical echo boxes."

Therefore, among the object of this invention are to provide an'echo box, the high Q 'of 'which is not seriously affected by the failure to meet close mechanical tolerances; to reduce the inechanical complications in the manufacture of echo boxes; to eliminate the necessity of 'afa'ctory adjustment or test bench by providing a more easily adjustable echo box; to provide an echo box whose adjustment is simple; to provide such a box whose calibration is more stable, more secure, and more reliable; to provide such a box in which it will not be necessary to make a difficult adjustment for the endplatesf to provide an echo box whose construction will produce an electromagnetic resonator, the ringing time of which is relatively insensitive to slight misalignment of the end plates; and to provide anecho box whose construction is comparatively 'sir'nple for manufacturing purposes.

Further objects, advantages, andnovel features of the invention will be apparent from the -description hereinafter contained, in which "reference is made to the drawingof a prefer-redembodiment of the invention'shown"in the" single figure of the drawing.

Referring now to the drawing, there is'illustrated in cross-section an echobox if) of circular cylindrical conformation as improvedin'accordance with'the teachings of the invention.

The section is taken'ina' plane through an; axle ll. Side wall or enclosing member "12 afforfds inner conductive surface 13 which is circula'r ly symmetric about axis ll. [Base member narfords an inner conductive surface l5 whichis planar and perpendicular toaxis H and the line elements of cylindrical surface l3.' Tritoth'dylindrical cavity [5 formed by cylindricalsurface l3 and .base surface leis inserted top member H, which thereby completely encloses cavity lt except for a small imiform annula-rjspage Qo i g it 3 I8 of the order of one or two hundredths of Wavelength between member I! and surface 13 and substantially concentric therewith. Cavity I6 is adjustable in depth by means adapted to move top member I! piston-wise along the direction of axis ll. Such means, as illustrated, may comprise a sleeve 20 held in fixed relation to wall IE to maintain the alignment of rod I9 and top member II; a spring 2! engaging a head 22 attached to rod l9 thereby urging head 22 firmly against caliper rod 23, which is threaded through caliper head 24, which, in turn, is attached to sleeve 20. It is clear that a reading of the advancement of caliper rod 23 into head 24 (such as may be made on the customary vernier which is not shown in the drawing) may be calibrated to read the depth h of cavity l6, and the consequent resonant frequency in a desired mode of oscillation.

One of the important features displayed in the drawing of Fig. l is the shape of the surface '25 presented to cavity i6 by top member ll. Surface 25 is spherical, of the order of a few hundredths of a wavelength in depth and of the order of three or four wavelengths in radius of curvature, the radius of curvature being colinear with axis of symmetry l I. In regard to other dimensions, diameter d of box H3 is somewhat less than one wavelength in the band of contemplated operating frequencies the inside height h of box is around two wavelengths. Depth or height it may be defined and measured as the distance from planar surface I to a plane substantially perpendicular to axis ll and parallel to surface [5 enclosing a cylindrical volume equal to the volume of cavity I6. Surface is a relatively small spherical deformation from the planar with a center of curvature substantially on the axis of cylindrical surface l3. Thus the entire spherical deformation consists of a spherical section which subtends an arc of roughly .25 radians. A slightly deeper spherical deformation subtending an arc of about .65 radians has given even better results than the one of longer radius of curvature in experiments conducted therewith.

The depth of the spherical portion, may be increased still further, but the optium location of feed orifice 26 may then have to be determined anew, perhaps by trial and error, as the energy concentrations of the fields are substantially altered.

In order to improve the Q of cavity l0, surfaces l3, l5 and 25 should be highly conductive, and for this purpose wall it and end pieces I 5 and I1 may be constructed of brass, and the surfaces may be carefully polished, buffed, and silver plated.

To transfer energy into or out of cavity H), a small aperture or window 26 may be located on a diameter of circular surface l5 for use in conjunction with a wave guide or coaxial cable. The window 26 is placed a distance from axis H for maximum coupling to the chosen operating mode. Other positions of the Window are possible which are highly efficient, for example in the side wall. The edge of aperture 28 may be bevelled as shown to improve the coupling. Member 20 may be attached to a fixed cover 21 forming what may be termed in the art a back cavity 28. Lossy material 29 may be utilized to minimize the effects of back cavity 28.

Before explaining the invention it is desirable to examine in part the theory of a cavity resonant to electromagnetic oscillations.

Modes of eletcromagnetic oscillation in cavities 4 are designated usually by a three-number sys-= tem, instead of the two-number system used for modes in wave-guides. The same cavity can oscillate at several different modes. The lowest frequency at which a cavity may resonate is usually termed the fundamental, and higher frequencies at which the cavity resonates are termed harmonics usually onl when integral multiples of the fundamental frequency. Cavity resonators may have various shapes, including shapes which result from taking sections of wave guides or coaxial cables and closing the ends. Since the flow of electrons is largely confined to an exceedingly thin layer of metal on the inner surface of the cavity, this layer should have low electrical resistance to avoid losses. In some cases, since only the conductivity of the inner surface is of importance, the cavity may be constructed of non-conducting materials with the inner surfaces painted or covered with a thin layer ofmetal or foil, or the structure may be of a metal of'lower conductivity coated or plated with a metal of higher conductivity, as cavity l0, wherein members l2, E4 and H are of brass, with surfaces I3, l5 and 25 silver plated.

For a cylindrical cavity, the modes are designated as TE or TM (transverse electric or magnetic respectively). One simple manner of expressing non-mathematically the subscripts indicating the conformation of the mode is that the first number indicates the number of whole (or full wave) patterns of the vector lines encountered in traversing a circumference of a circular cross section of the cavity, the second subscript indicates the number of half-wave patterns that are crossed in traversing a radius, and the third subscript indicates the number of half-wave patterns encountered in traversing the cavity in an axial direction, along its length or depth.

Actually, of course, the numbers indicating the modes of resonant oscillation of the cavity arise in the solution of the field equations because of the necessity of satisfying the boundary conditions which require that no electric vector can exist tangentially on a perfectly conductive surface.

For a more thorough discussion of the theor of electromagnetic oscillations in resonant cavities, refence may be made to Principles of micro-wave radio by E. U. Condon, published in Review of Modern Physics, volume 14, Numher 4, October 1942. Although the notation adopted herein may differ somewhat from that contained in the reference publication, the terminology adopted is the same, and, as pointed out in the publication, is borrowed from that of analogous problems in mathematical physics and particularly in quantum mechanics.

Naturally, different viewpoints may be adopted in investigating the theoretical aspects of echo boxes. One manner of attacking the problem is indicated in part by the article by E. U. Condon, to which reference is made above. There, for instance, the modes are ordered by a set of numbers, 11. However, there is no continuation of theory therein analyzing the problems involved in physical echo boxes in which the walls of the cavity have a finite conductivity, in which there are minute variations from the geometric figure adopted as a model, in which there is interplay between modes caused by the insertion of a coupling loop or other driving means into the cavity, etc. One way of continuing the attack on the problems involved is to set up a series of normalize'd orthogonal vector" wave" function's E; as in the article (which, as" may be shown, may be taken to represent the orientation ofthe'E vec-- may represent the E field or a series of'thefunc tions Ha with coeflicients ksqa maybe used to represent the H field. Thecoeffcients may be determined to satisfy specified boundary conditions.

as the field equations are satisfied term by term. These coeflicients and therefore each qa(t), may be determined explicitly by utilizing the orthogonal characteristics ofthe normalized functions in a manner analogous to that used in determining Fourier seriescoeificients. However, without explicitly determining at this point qa(t) as the amplitude function: of time associated with the 11th mode} it is possible: to write down the: set of dynamical Lagrangian equations.- govern-- ing' their behavior;

As an example-of the methods involved. in the use of Eag ran'gian equations, andthe significance thereof, reference maybemade to J. H. Jeans-,7

"Electricity" and Magnetismfl' cambridge University Press (1927'), chapter XVI. For a loss free cavity these- Lagrangian equations may; be

where'the subscript a labels all the'linearlyindependent modes oiZ oscillation and where,v as. customar-y, one identifies the? kinetic: energy 'I' with the: total magnetic energy of the system andthe potential encrgyV withthe totaleleo'tric? energy. Terms may; now be: provided to represent. externalexcitinig current,. dissipation losses int the walls of the cavity; andother departurcsfrom a loss fre'e geometrically perfect resonator.

on the; basis of the system of equations ob tainedby the theory, an equivalent circuit of a dissipative geometrically non-perfect cavity fed through a-co'axi'alline andLloop inputmaybe visualized as follows: Each normal mode maybe replaced by a closedresonant. mesh; consisting; of resistance, inductance. andcapacitancein series. Each mesh is coupled resistively' to all modesofa set for which these mutual resistancesadonot vanish, and similarly coupled; by mutual inductances": and elastances (inverse capacitances) and further each mesh in turn'is coupled toanaddh tional mesh representative of the external input circuit (if a coupling, loop isused', each is coupled to the additional mesh inductivelyother typesof coupling are possible) with certain coefilcients of mutual induction.

Bym'ea-ns of the theory developed in the manner outlined above, it is possible to show that in a perfect loss-free cylindrical-cavity there is no direct coupling between modes. walls are not perfectly conductive, so that. the cavity is not-loss' free, resistive coupling between pairs of modes may occur subject'to certain: se=

lection rules, as previously mentioned. In" particular; for TEU,m,n modes these selection rules indicate that such resistive coupling occurs only between modes whose" resonant frequencies never coincide either accidentally or otherwise;

This fact is important in the analysis since couplmg between modes only assumessi'gni'ii' fr? the cavity Cal a sources of" coupling" between certain pairs or modes: other sourcesmay be insulating gap such as l8'- between the end plate such as [1 of the side wan; such as I3 eccentricity" or mini-- uniformity" of radius" of an end platesuch as: either" I or bothend" platesif'b'oth do not: physicallycontact the side walls, resulting i'rr nonmnirormity ingaps such" as [8"; the existence of'a hack cavity, such as" zuwhich lies behindannular" gap and between members I15 and 21 any smalllellipt'icity or" variation from the ch cular" i'n thecross section or the cylinder? and; niostimportant'from the standpoint o'f the-iin provement accomplished by this invention, lackofexact' parallelism or'til'tin'g between end plates such as 14' and I1, especially where the latter has aplanar surface, instead of spherical sur face. such as25, presented to surface I5; If such surface is not planar; but has some other geemetricalj configuration; such as surface 25; tilt ing. of the? end plate" may be defined as the" exist enceiofan angle between an axisof symmetryof surface 25 and the axis of cylinder such as" H; ifsuch' axes are" in coincidence for geometricallyperfect" construction.

Itmaybefurthershowri that the TEOZin n modes in: cylindrical cavities are the only ones for whi'cli'lno" surface currents tend to flow" across a: gap such as [8 between the end plates such as I! andthe'cylindrical surface. such as l3. Therefore; the characteristic frequencies for thesemodes are little afiectedby these gaps or back cavities. highest Q values, other modes having lower Q va ues other things being" equal. In addition. other modes are greatly affected by the presenceof the gap andback cavities, because they, require current flow across the gap, thus caus ing'i coupling. to backcavity and other losses. For these reasons the oscillating modes of a tunable echo box are nearly always a. member of the TEt,m,n family of modes, usually with m equal to one.

In a perfect right circular cylinder, aii'y'TEb,m,n mode is degenerate with the companion pai'r TMl,m,n (even and odd) modes. so that a case of. triple. intrinsic degeneracy occurs. which. becomes of importance when considering the effect of a tilt of one of the end' plates such as IT. As; pointed out in the foregoing, the Qfs of the TMU,m,n degenerate modes are adversely afiecte'd, most particularly by gaps such as I8; The theory above advanced has indicated that a tilt, of: one of the end plates such as [1' results:- in coupling the degenerate modes with the operating TEO,m,n mode. If said tilt isconsidered'asthe vector sum of two component tilts mutually perpendicular, and the TM degenerate mode having an. axis of symmetry along one component tilt is considered odd, and along the" other one even, the tilt along one axis causes coupling to one of said degenerate modes and the tilt along the other axis causes coupling to the other degenerate mode. In the equivalent circuit the coupling is substantially inductive. It maybe shown both theoretically and: experimentally that: this" im Furthermore, these modes have theducti've coupling between modes of the same frequency is highly detrimental to the operation of the cylindrical resonator as an echo box.

As an anology, consider a complicated circuit consisting of many meshes, each series resonant, in which one mesh representing an operating mode is coupled to two other meshes representing the companion degenerate modes of the same resonant frequencies by mutual impedances, say inductances. There may be further coupling between these three among themselves and with other meshes in the circuit by other means such as mutual inductances, elastances (inverse capacitances), and resistances. However, the other meshes have resonant frequencies different from that of the meshes representing the operating mode and its companion degenerate modes. If the operating mode mesh is excited at its resonant frequency it is clear in a qualitative way that the two other meshes in which the greatest currents will flow will be the companion degenerate meshes which have the same resonant frequency. If, furthermore, the Qs of the companion degenerate meshes or one of them, is degraded considerably below that of the operating mesh, it is again clear in a qualitative way, due to the high currents excited in the resonant meshes of lower Q, that the Q of the entire circuit at this operating frequency is adversely aifected in an appreciable manner.

In previous echo box construction of the rightcircular cylindrical configuration it has been necessary in practice to make some provision for setting into extremely exact parallelism the planar surface of one end plate such as I and the planar surface of the opposite end plate. Provision may be made for such an adjustment by using a false bottom type of construction in which a set of screws move one end plate in adjustable tilting relationship to the side wall axis. Electrical tests were then made to determine the best adjustment and the adjustments usually were locked and sealed.

In this manner, the two end plates were brought into such nearly exact parallelism that coupling between the desired TEO,m,n modes and companion degenerate odd and even TMl,m,n modes was reduced. The reduction in coupling resulted in maintaining the Q of the circuit at a high figure.

However, if one of the end plates is deformed from a planar surface to a surface of spherical symmetry, such as surface 25, which has its axis of circular symmetry extending substantially along the axis of the cylindrical side walls, there is a shift in the resonant frequency of those modes which were companion degenerate modes in the undeformed cavity.

Returning for a moment to the theory, the total electric and magnetic energies inside the cavity are respectively:

T=mf zxlakbnq-gm-Hn sang;

a b a wherein La=/L0kaV henries and farads self-capacitances, due to a small variation from some simple geometrical shape, we may compute the increment of La from 2 L E a d1 from which:

2. kJV

Thus, having knowledge of the functions Ea and Ha for a given geometrical configuration, it is possible to compute the increment in La and Ca, at least to a first approximation, in a variation from the original configuration and consequently, at least to a first degree approximation, the change in frequency in an equivalent resonant mesh circuit having inductances and capacitances La and Ca respectively. Applying this process to the case of the spherical deformation from a planar end plate, it may be shown that with the total cavity volume unchanged the principal mode suffers no change in resonant frequency, but that the two companion degenerate modes do change in resonant frequency due to the deformation to a first degree approximation.

Referring again to the rough analogy introduced hereinbefore we see that whatever coupling exists between the resonant operating mesh and the meshes representing the companion degenerate modes is no longer a coupling between resonant circuits having the same resonant frequency. On the contrary, those meshes representing the companion degenerate modes now have resonant frequencies displaced from the resonant frequency of the main operating mode, with the result that lower currents are introduced in the two companion degenerate meshes. Therefore, a degraded Q in these two meshes no longer has the pronounced efiect in reducing the Q of the resonant operating circuit to which they are coupled.

Whatever may be the accuracy of the analogy or the analysis, it has been proved experimentally that an echo box having the configuration illustrated in the accompanying figure, instead of the configuration in which both end plates such as I? and I4 have planar surfaces presented to each other, has a longer ringing time, and generally improved characteristics.

In further marked distinction to the older type of echo boxes, the echo box of the invention displays a relative insensitivity to a tilting of the end plate surfaces. Therefore, it is not desired to bind the invention by any analogy or theory hereto discussed, which were introduced only as the probable explanation of the observed phenomena.

Although surface 25 has a spherical conformation, other types of deformation from the planar will accomplish the desired result. Any type of deformation which is circularly symmetric with the axis of the cylindrical surface of the cavity, that is, one in which a cross section along a plane generally perpendicular to that axis would show a circle as the curve of intersection with the inner surface of the deformed portion. Similarly, any deformation which may be approximated closely by the spherical type will cause the required improvement. Thus, the deformation may be paraboloidal, having an axis extending substantially along the cylindrical axis of the cavity.

It will be apparent to those skilled in the art that many variations of the invention may be made without departing from its scope and spirit. Therefore, it is not desired to restrict the invention except by the accompanying claims.

What is claimed is:

1. A cavity resonator having a desired mode of oscillation comprising: a hollow cylindrical first member; a second member having a plane surface closing one end of said first member, said plane surface being substantially perpendicular to the longitudinal axis of said first member; a third member movable relative to said first member extending into the other end of said first member, said third member having a concave surface facing said plane surface and symmetrically disposed about said longitudinal axis, the periphery of said concave surface being substantially contiguous with the inside surface of aid first member and in spaced relationship therewith, whereby the boundaries of said cavity resonator ar defined by said plane surface, said concave surface and the inside surface of said first member therebetween, said concave surface having a curvature which subtends an arc of no more than 0.65 radians; and said plane surface, said concave surface and the inside of said first member being composed of highly polished non-magnetic material having a high electrical conductance.

2. A cavity resonator according to claim 1, wherein said concave surface has a curvature which subtends an arc of substantially 0.25 radians.

3. A cavity resonator having a desired mode of oscillation comprising: a hollow cylindrical first member; a second member having a plane surface closing one end of said first member, said plane surface being substantially perpendicular 10 to the longitudinal axis of said first member; a third member extending into the other end of said first member, said third member having a concave surface facing said plane surface and symmetrically disposed about said longitudinal axis, the periphery of said concave surface being substantially contiguous with the inside surface of said first member and in spaced relationship therewith by an amount between 0.01 and 0.02 wavelengths at the operating frequency, whereby the boundaries of said cavity resonator are defined by said plane surface, said concave surface and the inside surface of said first member therebetween, said concave surface having a curvature which subtends an arc of no more than .65 radians; said plane surface, said concave surface and the inside of said first member being composed of highly polished material having a high electrical conductance; a fourth member closing the other end of said first member, a fifth member passing through said fourth member and attached to said third member for varying the distance between said concavesurface and said plane surface, said fourth member comprising glossy material substantially covering the inside surface thereof.

ALFREDO BAfiOS, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,233,263 Linder Feb. 25, 1941 2,245,627 Varian June 17, 1941 2,261,130 Applegate Nov. 4, 1941 2,269,456 Hansen Jan. 13, 1942 2,286,408 Hansell June 16, 1942 2,304,540 Cassen Dec. 8, 1942 2,323,201 Carter June 29, 1943 2,372,228 Schelkunoff Mar. 27, 1945 2,405,612 Schelkunoff Aug. 13, 1946 2,409,321 Stephan Oct. 15, 1946 2,410,109 Schelleng Oct. 29, 1946 2,489,075 Bishop Nov. 22, 1949 

